1. Field of the Invention
The present invention relates to a surface acoustic wave device, in particular, relates to a surface acoustic wave device having a base portion and a cap portion made of a resin.
2. Description of the Related Art
A surface acoustic wave device is widely used as a signal filter of an electrical and electronic device using an electromagnetic wave. The surface acoustic wave device is used as a transmitting and receiving filter of a radio communication device such as a mobile phone or a frequency filter for visual such as a television or a videotape recorder. In the surface acoustic wave device, a surface acoustic wave element is mounted on a package or the like. The surface acoustic wave element has a structure in which an electrode exciting a surface acoustic wave such as a comb electrode is provided on a piezoelectric substrate made of LiNbO3, LiTaO3 or the like. It is necessary to form a space on the piezoelectric substrate and on the electrode, because the surface acoustic wave propagates on a surface of the piezoelectric substrate. It is necessary to seal the surface acoustic wave element in order to gain trust, because there is not provided a protective membrane on the piezoelectric substrate and on the electrode.
There is a case where the surface acoustic wave element is mounted on a package made of a resin having a cavity, in order to reduce a cost of the surface acoustic wave device. In this case, the surface acoustic wave element is mounted on a base portion made of a resin, and the cap portion is adhered to the base portion so as to seal the surface acoustic wave element. The surface acoustic wave element is thus sealed in the cavity formed with the base portion and the cap portion.
Japanese Patent Application Publication No. 2002-171154 (hereinafter referred to as Document 1) discloses a surface acoustic wave device in which at least one of adhering faces of a base portion and a cap portion has a dull finish and adherence strength between the base portion and the cap portion is improved.
It is demanded that the base portion and the cap portion are not separated from each other, and an adherence between the base portion and the cap portion is resistant to a solder reflow, thermal shock of a soldering or a humidity. It is therefore demanded that the base portion is adhered to the cap portion at an adhering face. For example, adherence is obtained between components having a high affinity if the cap portion and the base portion are made of a thermosetting epoxy resin and the adhesive agent is made of an epoxy-based resin. It is however not possible to obtain a sufficient adherence strength.
With the art disclosed in Document 1, it is possible to improve the adherence strength between the base portion and the cap portion. However, an adhering area between the base portion and the cap portion is reduced, as the surface acoustic wave device is downsized. In this case, the adherence strength between the base portion and the cap portion is not sufficient even if the art disclosed in Document 1 is used.